Vacuum infusion or VARTM is a process used for moulding fibre composite mouldings, where uniformly distributed fibres are layered in one of the mould parts, the fibres being rovings, i.e. bundles of fibre bands, bands of rovings, or mats, which are either felt mats made of individual fibres or woven mats made of fibre rovings. The second mould part is often made of a resilient vacuum bag, and is subsequently placed on top of the fibre material. By generating a vacuum, typically 80% to 95% of the total vacuum, in the mould cavity between the inner side of the mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained herein. So-called distribution layers or distribution tubes, also called inlet channels, are used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases, the polymer applied is polyester or epoxy, and the fibre reinforcement is most often based on glass fibres or carbon fibres.
During the process of filling the mould, a vacuum, which in this connection is to be understood as an under-pressure or negative pressure, is generated via vacuum outlets in the mould cavity, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels, the polymer disperses in all directions in the mould cavity due to the negative pressure as a flow front moves towards the vacuum channels. Thus, it is important to position the inlet channels and vacuum channels optimally in order to obtain a complete filling of the mould cavity. Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin. Dry spots are thus areas where the fibre material is not impregnated, and where there can be air pockets, which are difficult or impossible to remove by controlling the vacuum pressure and possibly an overpressure at the inlet side. In connection with vacuum infusion, employing a rigid mould part and a resilient mould part in the form of a vacuum bag, the dry spots can be repaired after the process of filling the mould by for example puncturing the bag in the respective locations and by drawing out air for example by means of a syringe needle. Liquid polymer can optionally be injected in the respective locations, and this can for example be done by means of a syringe needle as well. This is a time-consuming and tiresome process. In the case of large mould parts, staff have to stand on the vacuum bag. This is not desirable, especially not when the polymer has not hardened, as it can result in deformations in the inserted fibre material and thus in a local weakening of the structure, which for instance can cause buckling effects.
Patent literature discloses examples of the use of a semi-permeable membrane, which increases the area in which the vacuum is active, and thus reduces the above problems. In this connection the term semi-permeable membrane means a membrane, which is permeable to gasses but impermeable to liquid polymer. Thus, if a semi-permeable membrane is placed across the fibre insertion, air pockets can be removed more easily or prevented entirely. Furthermore, it is known from WO 06/058541 to let some of the channels function sequentially as vacuum channels and inlet channels, thereby being able to control the flow fronts to a high degree and even being able to reverse the flow fronts to remove dry spots. The channels can thus first be used to evacuate the mould cavity by connecting them to a vacuum source and later be used as resin inlets by interrupting the connection to the vacuum source and connecting them to a polymer source.
Typically, the composite structures comprise a core material covered with a fibre reinforced material, such as one or more fibre reinforced polymer layers. The core material can be used as a spacer between such layers to form a sandwich structure and is typically made of a rigid, lightweight material in order to reduce the weight of the composite structure. In order to ensure an efficient distribution of the liquid resin during the impregnation process, the core material may be provided with a resin distribution network, for instance by providing channels or grooves in the surface of the core material.
As for instance blades for wind turbines have become still bigger in the course of time, and may now be more than 60 meters long, the impregnation time in connection with manufacturing such blades has increased, as more fibre material has to be impregnated with polymer. Furthermore, the infusion process has become more complicated, as the impregnation of large shell members, such as blades, requires control of the flow fronts to avoid dry spots, which control may e.g. include a time-related control of inlet channels and vacuum channels. This increases the time required for drawing in or injecting polymer. As a result the polymer has to stay liquid for a longer time, normally also resulting in an increase in the curing time.
Furthermore, the VARTM method is encumbered with the problem that the vacuum during the evacuation process compresses the fibre layers, which therefore may wrinkle or bulge due to the vacuum level varying throughout the mould during the beginning of the evacuation process. Therefore, the composite structure, such as a wind turbine blade, may also contain wrinkles after impregnation and curing, which in turn can lead to reduced performance of the blade or result in local weak points, which may cause breakdown of the blade during use.